Conventional techniques for forming valve seats in cylinder heads involve cryogenically freezing sintered alloy iron annular seating rings, force fitting the rings into recesses in the cylinder head, and then machining. Recently it has been proposed to reduce the iron sector of the seating rings and it has now been proposed to electrically weld or otherwise bond the seating rings into position using resistance welding techniques, typically DC current derived from a single or more preferably multiphase AC supply.
Problems have arisen in designing apparatus for achieving this proposal, not least of which involve the handling of the relatively small metal rings which are to constitute the seatings, and the positioning of these reliably for welding.
An ancillary problem arises from the inevitable process spatter which, if not removed from the cylinder head, would cause problems when the resulting engine was run.
In EP-A-0751284 there is disclosed a method of securing a valve seat member to a cylinder head recess, in which the cylinder head is placed on a press machine with the recess facing upwards, the member is pressed down against the recess, and the member is metallurgically bonded to the recess by applying a voltage thereacross via upper and lower electrodes. However, this prior disclosure does not obviate either of the above mentioned problems.